1. Field of the Invention
The present invention relates to a piston machine and more particularly to a hydraulically actuated piston machine with a cylinder, and a piston turnably connected with a piston rod and axially sliding in the cylinder.
2. Description of the Prior Art
Piston machines of the above-mentioned general type are known in the art. In one such piston machine design in accordance with the piston-piston rod principle (Projektblatt RM 1-002 of the company G. Dusterloh, G.m.b.H. 4322 Sprockhovel, Federal Republic of Germany), the piston force is transmitted via a cylindrical pin to the piston rod. The pin is fixedly connected with the piston and slides in an opening which is provided in a piston rod shaft transversely to the directional movement of the piston and is a so-called piston rod eye. The piston rod shoe abuts on an eccentric of the crank shaft.
Piston machines of this construction have the advantage that relatively low friction forces take place and also the relative speeds of the structural elements in sliding contact are substantially uniform at all locations. Finally the wear in the contact zones is approximately uniform. The disadvantage of this construction is, however, that more or less unavoidable failure in the parallelism of the central axes of the piston rod eye, on the one hand, and the crank shaft, on the other hand, cannot be compensated. Thereby it is impossible to provide an unobjectionable guidance. By inclined running there is increased leakage as well as a non-uniform wear. This can lead to operational disability because of, for example, piston seizure.
For avoiding the above mentioned deficiencies extremely high manufacturing accuracy is required.
In a piston machine in accordance with the another embodiment (Magazine "Asr-Digest Fur Angewande Antriebstechnik" Vol. 10, October 1975, page 28, FIG. 2), the piston force is transferred via a spherical joint to the piston rod. Here a ball is arranged on the piston rod shaft and slides in a piston recess corresponding to the ball. The advantage of this construction is that the parallelism error cannot here have any negative influence, since the joint can be adjusted in all directions freely about its center. The manufacturing requirements are therefore lower. However, as compared with the cylindrical connection, higher friction forces arise. These friction forces are especially significant when the ball is flattened in the region of the central axis of the piston rod to allow passage of pressure medium to an unloading field. In this case the piston force can be transferred only over a spherical ring-shaped contact zone. As a result of this high centrally acting forces are provided which consequently cause high friction forces. The efficiency of the piston is considerably decreased. Moreover there is here also the disadvantage that because of the different relative speed in the contact zone, a non-uniform wear of the spherical surfaces is caused.
Both above-described construction of the piston machines in accordance with the piston-piston rod principle are characterized by the fact that the joint especially in highly loaded machines is subjected to high surface pressure. The resulting high wear and reduced service life cannot be avoided by the utilization of extremely high loadable materials. Furthermore, it is also of the disadvantage that with high wear, the piston, the piston rod and the piston pins must be replaced, while a repair of the worn hinge is not possible or in some cases cannot be done with reasonable expenses. Finally, another disadvantage is that in both above-described embodiments the radial forces take place which are effective for progressive wear in direction of widening the piston rod opening or the piston skirt.